1. Field of the Invention
The present invention relates to luminance control of an aperture grill type display device.
2. Description of the Prior Art
FIGS. 9A-9C are diagrams for explaining mechanisms of a color image display in the conventional CRT display device 9. FIGS. 10A and 10B are diagrams showing examples of a display picture SN9 displayed on a display screen of the CRT display device 9. A CRT portion (a Braun tube) of the conventional CRT display device 9 comprises an electron gun 91, a color discrimination mechanism 92 and a fluorescent surface 93. The fluorescent surface 93 includes stripe fluorescent films 93r, 93g and 93b that are arranged in turn and are elongated in the vertical direction of the CRT display device 9.
The electron gun 91 includes three cathodes 91r, 91g and 91b, which emit electron beams 8r, 8g and 8b, respectively. There is a type of the electron gun 91 that has one cathode emitting three electron beams 8r, 8g and 8b. The emitted electron beams 8r, 8g and 8b hit the fluorescent films 93r, 93g and 93b, respectively. Then, red, green and blue light rays are generated from the portions hit by the electron beams, respectively. Various kinds of colors can be reproduced by adjusting intensity values of the electron beams 8r, 8g and 8b. While adjusting the intensity values, the electron beams 8r, 8g and 8b are deflected so as to scan the entire fluorescent surface 93. Thus, a color image can be displayed on the entire screen of the CRT display device 9.
The color discrimination mechanism 92 is used for leading the electron beam 8 to hit a predetermined position on the fluorescent surface 93 with precision. A shadow mask or an aperture grill is used as the color discrimination mechanism 92. As shown in FIG. 9B, the shadow mask is a metal plate 92a with many openings 92h arranged regularly. Namely, only electron beams 8 that passed through the openings 92h can hit the fluorescent surface 93. Thus, it is possible for the electron beam 8 to hit a predetermined position precisely.
However, there are some problems when using the shadow mask. One of them is that luminance drops as departing from the center of the fluorescent surface 93. Another problem is that heat generated by energy of the electron beam 8 may deform the shadow mask so that an image can be blurred as a whole.
As shown in FIG. 9C, the aperture grill comprises many wires 92b arranged at a constant narrow pitch. Each of the wires 92b is stretched by tension in the vertical direction (in the length direction). Some of the emitted electron beams 8 pass openings 92h′ between neighboring two wires 92b so as to hit the fluorescent surface 93. Thus, the electron beam 8 can hit a predetermined position precisely.
The usage of the aperture grill can solve the above-mentioned problem of the shadow mask. Namely, since the aperture grill has the openings 92h′ that are larger than the openings 92h of the shadow mask, more electron beams 8 can hit the fluorescent surface 93, so that images can be displayed more clearly.
In addition, even if the wire 92b is elongated by the energy of the electron beams 8, the positions of the openings 92h′ are not changed since the wire 92b is always stretched by the tension in the vertical direction. Therefore, images do not become blurred as distinct from the shadow mask.
However, when using an aperture grill, vertical stripes can appear in the display picture. For example, it is supposed that a display picture SN9 indicating a state of opening a vertically elongated window WD9 on a desktop DS9 of an operating system (OS) is displayed on the screen of the display device 9 as shown in FIG. 10A.
If there is not large difference between luminance values in the entire display picture SN9, vertical stripes do not appear in the display picture SN9. It is because that there is not large difference between elongation ratios of the wires 92b due to the energy of the electron beams, so that all the wires 92b are stretched by the tension in the vertical direction and the positions of the openings 92h′ are retained at the proper positions.
However, when the luminance of the window WD9 is increased to be higher than the luminance of other areas, i.e., the desktop DS9, vertical stripes can appear in the portion of the window WD9 as shown in FIG. 10B. The reason of this is as follows.
When electron beams having high energy for displaying the window WD9 hit a specific wire 92b (hereinafter, referred to as the wire 92bw) in a concentrated manner, the wire 92bw may become longer than other wires (hereinafter, referred to as the wire 92bd). If the difference between the elongation ratios is too large, the tension in the vertical direction and the restoration force of the wire 92bd can be balanced when the wire 92bd is stretched to some extent by the tension in the vertical direction. Then, the tension in the vertical direction cannot act on the wire 92bw, which may remain sagged. Thus, the wire 92bw can vibrate or be entangled in a neighbored wire 92b so that the position of the opening 92h′ can be changed. Thus, the electron beams cannot hit the fluorescent surface 93 properly, so that the vertical stripes can appear as shown in FIG. 10B.